1. Field of the Invention
This invention relates to a vertical scan circuit for a raster-scanned cathode ray tube (CRT) display, in particular to a circuit which allows automatic picture height adjustment and centering for different screen formats.
2. Description of the Prior Art
In raster-scan CRT video displays, an electron beam(s), is deflected and modulated to produce an image on the screen. Horizontal and vertical scan frequencies refer to the rate at which the beam can scan a single horizontal line or a complete screen respectively: bandwidth indicates how fast the beam can be turned on or off. These performance indices dictate the resolution of the display. Particular functions require different resolutions or screen formats, that is the number of characters or picture elements (pixels or pels) per line or horizontal lines per screen. The majority of systems are designed such that an individual display is configured for a particular screen format. However, many standards have evolved, and attempts have been made to provide one display capable of presenting data in several of such formats.
The NEC Multisync II (trademark) is an example of a multimode display, which can "lock into" a range of formats: however, picture height and centering need to be reset manually for each one. U.S. Pat. No. 4,574,279 describes a multiple format system in which horizontal and vertical scan frequencies are generated such that one of two formats can be selected, the same picture size being maintained by the application of one of two preset voltages to height adjustment circuits IBM Technical Disclosure Bulletin Vol. 28, No. 12, pages 5603-4, illustrates a method of increasing the number of vertical resolution modes of a display without altering the horizontal scan rate: the width of the video display enable signal, which corresponds to the number of horizontal lines displayed, is converted to a resistance value and used to control the vertical position of the beam.
Whilst it is clear, therefore, that multiformat displays do exist, nowhere in the prior art is the function fulfilled of automatic picture height and centering for a wide range of non-preset screen formats.
In raster video displays, the vertical position of the electron beam is controlled by the generation of deflection voltage signals, the voltage value varying with time. A change in format could involve, for example, varying the duration of the active video time when data is actually being displayed on screen.